1. Field of the Invention
The invention relates generally to modems and more particularly to a mechanism for changing the modem technology that is selected by a pair of modems to be used as the basis for communications between the modems.
2. Description of the Relevant Art
In order to establish communications between two modems, it is necessary to define a framework within which the modems will communicate. This communication framework is related to the technologies employed by the respective modems. The process of selecting the proper framework is referred to as inter-working or handshaking. At the beginning of the handshaking process, the modems select a particular modem technology which will be used in the communications. Although a modem may employ a modern modem technology, most modems implement older technologies as well. This allows a given modem to be compatible with other modems which use the older technologies.
The selection of the modem technology to be used in communications between a pair of modems is referred to herein as negotiation. During the negotiation stage, one modem identifies the technology of the other. If one modem uses a newer technology, this modem will detect the older technology of the other modem and "fall back" or use the older technology. In this manner, the modems will typically select the most recent technology which is common to both modems.
Each modem technology allows communications to be conducted at a range of speeds, or bit rates. Typically, more recently developed technologies have higher allowable bit rates than older technologies. One of the bit rates allowed by the selected technology is selected by the modems for the connection. The bit rate selected by the modems is a function of the quality of the telephone line used for the connection--the more noise on the line, the slower the bit rate that will be chosen. The selection of the appropriate bit rate is referred to herein as training.
The handshaking process thus consists of a negotiation to select a modem technology and training to select the highest speed allowed by the selected technology and the line quality. At the conclusion of the handshaking process, the modems can begin transmitting data to each other (although it is not necessary that data be transferred--the modem connection may have been established in anticipation of future data transfers.)
Some modems are configured to re-enter the training stage if the condition of the telephone connection changes during communications between the modems. This process is referred to herein as re-training. While re-training can be used to select a speed suitable to the changed condition of the telephone line, the speed must be one allowed by the previously selected modem technology. The selected modem technology cannot be changed during re-training. In order to select a different modem technology, the telephone connection must be dropped and re-established, after which the entire handshaking process must be repeated. Changing the modem technology in the prior art thus incurs all of the overhead associated with establishing a new connection. There is also the possibility that there may not be a line available or that the quality of the connection may have more noise than the previous connection.